1. Field of the Invention
The present invention relates to a hydraulic pressure control system for a belt-type continuously variable transmission suitable for continuously changing the speed ratio by means of a belt member passed over two variable pulleys each having a belt groove with a width that is variable according to a hydraulic pressure applied to the pulley.
2. Description of Related Art
Such a hydraulic pressure control system for a belt-type continuously variable transmission is disclosed, e.g., in JP 11-82725A or JP 2001-263474A, of which the latter became publicly available after the priority date of the present application. In the hydraulic control system as known from these patent documents, when the speed ratio of the belt-type continuously variable transmission is changed to the high speed side, it is judged whether or not the desired change is actually attained. If it is judged that that the desired change of the speed ratio to the high speed side is not attained, the line pressure is increased until the desired change is attained. Typically, the line pressure is set to a low value in order to eliminate drawbacks caused by a high line pressure, such as energy loss due to excessively high hydraulic pressure discharged from an oil pump, or degraded fuel consumption due to frictional loss between the belt member and the variable pulleys, as a result of excessive force with which the belt member is clamped by the variable pulleys.
However, since the line pressure is usually set with some margin in consideration of dimensional fluctuation of the components, even when a desired speed ratio as determined according to an operating condition is attained, there may be instances wherein the hydraulic pressure control system is supplied with a line pressure that is higher than the minimum pressure allowing power transmission by the belt member.